The U.S. and Russia are backing separate campaigns to wipe out the Islamic State militant group (ISIS) in its final stronghold of Deir ez-Zor, in eastern Syria. As the rival powers and their allies close in on jihadists, however, an international rivalry escalates, and seemingly natural obstacles grow increasingly suspect.

With support from Russia and Iran, the Syrian military has broken a three-year ISIS siege on fellow soldiers trapped in Deir ez-Zor and beaten the militants back across the Euphrates river that divides the city. As Syrian troops advanced Tuesday, however, Moscow has accused the nearby U.S-led coalition campaign against ISIS of deliberately raising the river's water levels using dams located within territory held by the U.S-backed Syrian Democratic Forces, a mostly Kurdish coalition of Arabs and ethnic minorities, to frustrate the Syrian military offensive.

"The water situation on the Euphrates has deteriorated dramatically in the past 24 hours. As soon as the Syrian government troops began to cross the river, the water level in the Euphrates rose within hours, and the current velocity nearly doubled to two meters per second," Russian Defense Ministry spokesperson Igor Konashenkov told the state-run Tass, the Russian news agency.

"Since there have been no rains, the only source of such changes in the water situation is man-induced water discharge at dams upstream the Euphrates. These facilities are held by opposition groups controlled by the U.S.-led coalition," he stressed.

A Syrian army soldier takes a picture of the Euphrates river in al-Bugilia, north of Deir ez-Zor, Syria, on September 21. Omar Sanadiki/Reuters

The U.S.-led campaign, known as Combined Joint Task Force—Operation Inherent Resolve, told Newsweek in a statement Thursday that "a number of factors will impact the water levels in the river, but we are not aware of any direct intervention by the coalition or our partner forces." The People's Protection Units, a Kurdish militia that forms much of the Syrian Democratic Forces, also denied any involvement and said it "did not notice any water level change" in a statement sent Friday to Newsweek.

Meanwhile, "mystery" fires broke out Thursday in the nearby Koniko gas field, the largest of its kind in Syria, according to the Syrian Observatory for Human Rights, a U.K.-based monitor with ties to Syria's exiled opposition. The group said it had not yet determined the source of the fires but noted that the Syrian military was clashing with ISIS a little more than three miles away, and the Syrian Democratic Forces were posted about eight miles away. The state-run Syrian Arab News Agency reported Tuesday that Syrian troops were attempting to take the field, but neither side has claimed responsibility for the fires.

The U.S.-led coalition confirmed to Newsweek Thursdaythat the U.S. and partner forces were "aware of the fires in the vicinity of the Koniko gas field" but denied playing any role in them, or having any knowledge of how they started or what was being done to extinguish the blaze.

Smoke rises as Syrian army soldiers stand near a checkpoint in Deir ez-Zor, Syria, on September 21. Omar Sanadiki/Reuters

Mistrust between the dueling campaigns by the world's two leading military powers recently spilled into violence. After the Syrian Democratic Forces said Saturday it had come under fire by the Syrian military with Russian air support, Russia accused the Syrian Democratic Forces of firing on its own positions Thursday. Both sides deny involvement in perpetrating either incident, but Konashenkov said that any further provocations would "be immediately suppressed with all military means."

Russia also accused the U.S. Wednesday of coordinating with jihadists of Hayat Tahrir al-Sham, a group once known as the Nusra Front and still widely believed to retain its Al-Qaeda ties, for a surprise assault in the northeastern rebel-held governorate of Idlib. The U.S. once supported various armed groups trying to overthrow Syrian President Bashar al-Assad since 2011, but after a rise in jihadist activity, Washington switched gears to the Syrian Democratic Forces.

The Syrian government and the Kurd-dominated Syrian Democratic Forces both oppose ISIS, other jihadists and Turkish-backed rebels still trying to oust Assad, but disagree over the path to stabilizing the war-torn country. Kurds, bitter over what they consider to be political and cultural oppression by Assad's government, seek greater autonomy in the north, while the Syrian government and its supporters have criticized the Kurds' coordination with the U.S. and have called for absolute power to be restored to Assad.